1. The Basic Properties Of Powder Materials

With the rapid development of the lithium-ion battery industry, there are more and more safety problems in the use of batteries, in which the material problem is a major problem that can not be ignored, the selection of materials and the composition of the system of ratios determines the safety performance of the battery cell. In the selection of cathode and anode active materials and diaphragm materials, manufacturers do not monitor the characteristics and matching of raw materials, the battery will have more safety hazards. In the current development process of battery cells, the overall quality control of powder materials has also been paid attention to, in which the compaction density index is also a key indicator affecting the performance of the battery, and the level of compaction density is closely related to the size and distribution of particles and other parameters of the key materials, such as positive and negative electrode powder, and is closely related to the capacity, the internal resistance of the battery, and the battery life, etc. The compaction density measurement has been widely used by the manufacturers in the development of battery cells. Compaction density measurement has been known and recognized by materials companies, cell companies, universities and research institutes, but there are relatively more influencing factors in the measurement process, which need to be further analyzed systematically to determine the appropriate parameter conditions to complete the systematic measurement.

According to Mr. Yang Shaobin, “lithium-ion battery manufacturing process principles and applications,” described in the book, the powder density is usually considered to be the total mass of the powder sample per unit volume, the powder density has three forms, namely, filling density, particle density and true density. Among them, particle density, also called apparent density, contains the particles themselves and the micropores within them, excluding the inter-particle voids. The true density is mainly the total volume of the powder and does not include the sum of the true volume of the micropores within the particles and the voids outside the particles. The filling density corresponds to the total volume of the powder containing the overall inter-particle voids and the micropores inside the particles, also known as the heap density. The order of magnitude between different densities is: true density > particle density > filling density^[1].

Filling density includes loose-fill density, vibration density and compaction density. Loose packing density is the density of free accumulation of particles under no pressure conditions; vibration density is mainly the filling density tested after vibration of the powder; compaction density is the overall filling density of particles after external pressure. Compaction density is the overall filling density of the particles after external pressure. The order of comparison of the size of the filling density is as follows: compaction density＞vibration density＞loose loading density. Electrode compacted density pool is one of the key indicators of concern in the design process of lithium-ion batteries, electrode compacted density = surface density / (thickness of the electrode rolled – the thickness of the collector), and the compacted density of the powder material = the mass of the powder after compression / the volume of the powder after compression; the determination of the compacted density of the powder can be used in the powder research on the differences in compacted density of powders under different conditions of the process modification to carry out an effective assessment, and at the same time In the powder production process stability and incoming material monitoring are of great significance^[1].

Powders have similar fluidity to liquids, similar compressibility to gases, and specifically the deformation resistance of solids. Powder study is mainly a science based on the properties of various shaped particle aggregates. Most of the particle size of powder research between 0.1 ~ 100μm, a small number of particles can also be as small as 1nm or as large as 1mm. powder compression process will be subject to the powder particle size and its distribution, morphology, density, specific surface area, void distribution, surface properties, mechanical properties and mobility properties, and ultimately show the filling properties and compression performance of different. Lithium-ion battery production and manufacturing process of the electrode roll pressing process process is actually the process of compaction of cathode and anode electrode materials, is the rearrangement of the powder and densification process (Figure 1 for the electrode coating roll pressing process of microstructure evolution schematic), so the powder performance research is also the focus of the current lithium-ion battery process modification research and development. In this paper, the actual determination process of lithium powder compaction density as the basis for systematic analysis, clear impact on the powder compaction density, compression performance measurement and parameter selection of correlation index, in order to ensure the effectiveness and rationality of the compaction density determination and assessment^[1,2].

Figure 1. (a) Schematic diagram of the microstructure evolution of the coating material during the roll pressing process of the cathode (b) anode electrode sheet^[2]

2. Powder Filling & Compression Performance

After the powder is compressed by external force, under small pressure conditions, the filling between the powder particles is not tight, and the porosity between the powders is large; with the increase of the external force, the powder particles flow and rearrangement to form a tight stacking state, and the porosity between the particles is also reduced; as the pressure continues to increase, the powder particles undergo elastic deformation, and the change in the porosity between the particles is not significant, but the particle pore size will be reduced; along with the further increase of pressure With the further increase of pressure, some powder particles will be irrecoverable plastic deformation, particle pore size will be further reduced; at the same time will be accompanied by brittle particles system breakage, particle pore size will be significantly reduced. The actual compression process of powder is a complex composite process, elastic deformation will coexist with plastic deformation, elastic deformation can be recovered, plastic deformation is partially unrecoverable^[1].

Powder compression performance is the focus of the study of powder materials mechanical properties, in the field of pharmacology has a relatively comprehensive study, and in the field of lithium-ion batteries we are more concerned about the compression performance of the finished battery tends to be, with the development of lithium-ion battery industry and the powder material compaction density indexes are valued, the compression performance of powder materials is also gradually being focused on by the researchers, and more and more researchers prefer to be from the powder, the electrode, More and more researchers prefer to evaluate the compression performance of powder, electrode, cell and other layers to determine the relevance of each stage of the process development process. IEST production of the PRCD series of powder resistance & compaction density tester currently has nearly 200 + customer groups in the lithium industry, currently used for powder materials process modification indicators to assess the difference between the evaluation and batch stability assessment of an effective means of the instrument in addition to the basic resistance and compaction density indicators in addition to the determination of the powder material compression properties can also be realized.

As shown in Figure 2 shows the PRCD series of powder resistance & compaction density test equipment, Figure 3 shows the compression performance test function schematic diagram, which (a) & (b) for the assessment of compression performance of the unpressurized test method, powder particles are subjected to pressure accompanied by elastic deformation and plastic deformation, when the pressure exerted on the powder particles to unloading, the elastic deformation part of the restoration, combined with the pressure setting mode in Figure 2 (a), the thickness of the powder after the pressure is unloaded. Deduction of the pressurized powder thickness is defined as the rebound thickness of the powder, Figure 2(b) shows the rebound thickness difference between different materials with the pressure change curve, with the increase of the pressurized pressure, the rebound thickness of the material gradually increases and tends to stabilize. Combined with the mechanism of the powder compression process, when the powder itself is broken, the irreversible plastic deformation accounts for a larger proportion, and the rebound thickness of the material will not be able to recover after unpressurization, which is the original intention of the development of unpressurized test method, hoping to realize the characterization of powder particle crushing through the unpressurized test mode. Figure 2 (c) and (d) show the steady state test pressure application mode and steady state test results, the method mainly characterizes the powder compression stress – compression thickness deformation percentage curve. In (d), ① is the maximum deformation point of the material after compression, with the unloading of the pressure, ② is the irreversible compression deformation part of the material, ① – ② is the reversible deformation part of the material after pressurization and unloading of the material, the results of the experiment for powder materials with different particle sizes or ratios of the particles will be significantly different, and this method can be combined with the method of assessing the stress-strain properties of the material in the actual development of the powder material.

Figure 2. IEST Powder Resistivity & Compaction Density Measurement System (PRCD3100)

Figure 3. Stress and strain curve during the pressurized pressure relief of the three NCM materials

3. Powder Compaction Density

Powder materials compression process will be accompanied by changes in the voids between the powder as well as the particles themselves, which can be used in the Heckel equation to express the relationship between the void ratio and the compression pressure, it is more to summarize the compression force and density changes in the semi-empirical formulas, the void ratio (1) and the Heckel equation expression (2) is as follows^[4]:

Figure 4. Void ratio (1) and Heckel equation expression (2)

Where ρb is the filling density, ρt is the true density, p is the pressure; D is the relative filling density of the powder when the pressure is p, and the void ratio = 1-D, k and A are constants, which can be obtained from the slope and intercept of the straight line part of the empirical formula. the significance of A can be clarified by combining with the formula A= ln[1/(1-DA)], where the relative density DA is the maximum density of the particles after rearrangement under the low pressure and before the deformation. The relative density DA is the maximum density before deformation after particle rearrangement under low pressure, and the value may be closely related to the compaction density of lithium-ion battery wafer level; k is a parameter to measure the size of powder plasticity, the larger the value of k is, the larger the density change caused by the same pressure is, and the larger the plasticity of the powder material is. Powder compression is a very complex process, Heckel’s equation is usually applied to high-pressure, low void ratio powder material.

Lithium-ion battery design and manufacturing process in the current powder compaction density assessment has become the focus of many materials and host plant indicators, the stability of the powder compaction density determination is particularly important, the determination of the powder compaction density is actually the ratio of the total mass of the tablet and the total volume of the tablets after the compression is different pressure compression of the powder after the filling density of the actual determination process of people, machines, materials, law, the environment and so on are all the key indicators of the impact of the determination. In the actual determination process, man, machine, material, method and environment are the key indexes affecting the determination. The national standard GB / T 24533-2019 in Appendix L provides a program for the determination of powder compaction density, which is mainly combined with a manual tablet press to pressurize the powder samples after manual thickness measurement to obtain the thickness of the powder after compression, and then calculate the compaction density of the powder, the thickness part of this standard method is pressurized after the completion of the pressure is unloaded after the pressure exerted on the powder end of the test, in fact, similar to the Figure 3 (a) In fact, it is similar to the unpressurized test method in Figure 3(a). With the increase of attention to the compaction density, there are more and more professional testing equipments for compaction density determination. Compared with the method of tablet press assisted testing, the current multi-autonomous pressurization and thickness measurement integrated equipment, equipped with a stable lower computer control system, sends the parameter commands through the software system of the upper computer, which can effectively improve the overall testing efficiency.

With reference to the current testing capacity of different laboratories, the compaction density test mainly includes single-point unpressurized test, variable pressurized multi-point test, variable pressurized unpressurized continuous test, etc., such as Figure 4 for the compaction density of different powder materials under variable pressurized conditions, the process is accompanied by the continuous pressurization of the powder material, which is closely related to the compression performance of the powder. Compaction density index in the R&D application of the choice of variable pressure conditions for the determination, combined with the size of the powder particles, particle size distribution, specific surface area and void ratio and other indicators to do further analysis; also can be combined with the performance of the post-process section of the relevance of the assessment. In addition, compaction density in the application of batch stability monitoring, not inevitably involves different manufacturers of different types of equipment results against the standard, compaction density measurement itself will be pressurized with the equipment, thickness measurement, test mold size selection, sampling volume and other indicators are closely related, if the standard needs to be further clarified for the correlation of the indicators, and ultimately determine the effective benchmarking parameters; if it involves a large difference in the functionality of equipment If there is a big difference in the function of the equipment involved, the absolute difference between the test results of different equipment can be tested to make clear the difference in testing capabilities after benchmarking; in short, the parameter difference is very important to clarify the parameters of the first test comparison, in order to prevent the waste of time and cost.

Figure 4. Determination of compacted density of different powder materials under variable pressure conditions

4. Summary

Powder materials compression performance and compaction density are closely related to each other, and powder compaction density index is also a key indicator affecting the performance of the battery, the level of compaction density is closely related to the key main material, such as cathode and anode electrode powder particle size and distribution of other parameters, and is closely related to the capacity, the internal resistance of the battery, the battery life and so on, and the assessment of the compaction density has a very important significance.

5. References

[1] Yang Shaobin, Liang Zheng. Lithium-ion Battery Manufacturing Process Principles and Applications.

[2] mikoWoo@Ideal Life. Theory and Process Basis of Lithium-ion Battery Polar Cells.

[3] B K K A , A S A , A H N , et al. Internal resistance mapping preparation to optimize electrode thickness and density using symmetric cell for high performance lithium-ion batteries and capacitors[J]. Journal of Power Sources, 2018, 396:207-212.

[4] SI Guo-ning, HUANG Wan-ting, LI Gen-sheng, XU Fei, CHU Meng-qiu. Application Research of Different Compression Model on Four Powder Excipients Compression[J]. Chinese Pharmaceutical Journal, 2018, 53(23): 2021-2028 https://doi.org/10.11669/cpj.2018.23.009